Add backports repos to base and builder images

These don't install anything by default, but allow people
to easily opt-in to a backport package if they need to without
lots of extra lines of boilerplate.

The base python image is on bullseye now instead of buster. That
means the libffi version is 7 not 6.

Change-Id: I0e0c2669d838fb622422f696f73e96e409157270

docker/python-base/Dockerfile

@@ -23,7 +23,8 @@ RUN echo 'APT::Install-Recommends "0";' > /etc/apt/apt.conf.d/95disable-recommen
 RUN apt-get update \
   && apt-get install -y dumb-init \
   && apt-get clean \
-  && rm -rf /var/lib/apt/lists/*
+  && rm -rf /var/lib/apt/lists/* \
+  && echo 'deb http://deb.debian.org/debian bullseye-backports main' >> /etc/apt/sources.list
 
 # Upgrade pip to fix wheel cache for locally built wheels.
 # See https://github.com/pypa/pip/issues/6852

docker/python-builder/Dockerfile

@@ -36,7 +36,8 @@ COPY scripts/install-from-bindep /output/install-from-bindep
 RUN dpkg -i /tmp/python3-dev_4.0.0_all.deb \
   && rm /tmp/python3-dev_4.0.0_all.deb \
   && pip install --no-cache-dir bindep \
-  && rm -rf /var/lib/apt/lists/*
+  && rm -rf /var/lib/apt/lists/* \
+  && echo 'deb http://deb.debian.org/debian bullseye-backports main' >> /etc/apt/sources.list
 
 # Upgrade pip to fix wheel cache for locally built wheels
 # See https://github.com/pypa/pip/issues/6852

docker/uwsgi-base/bindep.txt

@@ -1,5 +1,5 @@
 gcc [compile]
 libc6-dev [compile]
 libffi-dev [compile platform:dpkg]
-libffi6 [platform:dpkg]
+libffi7 [platform:dpkg]
 libssl-dev [compile platform:dpkg]